Process control systems control an industrial process by means of various field devices, e.g. regulating devices, control devices, sensors, transmitters and the like, which are connected to the process. A typical field device is a control valve provided with a valve controller. A field device is typically controlled by a process controller using a suitable control algorithm on the basis of the measurement results (feedback) obtained from the process and the set values. Thus, a so-called control loop is formed. An example of a closed-loop control is shown schematically in FIG. 1. A control loop may comprise e.g. a process controller 2 controlling a field device in a process 4 and a measured feedback from the process 4 to the process controller 2. Measurement sensor/transmitter 6 measures the process variable (temperature, pressure, level, flow, or analysis) and converts the measured process variable to units useful to the controller (psig, kPa, etc) or some electronic value such as 4-20 mA. The measurement sensor/transmitter 6 may be connected directly to the process 4 and transmit the variable to be controlled to the controller 2, usually some distance away, e.g. over a wired or wireless connection or network. The controller 2 makes a comparison between the desired control point yset (set point) and the measured control point ymes and reacts to the difference, deviation or error according to a preset control action or algorithm. For illustrative purposes, the comparison function 8 and the resulting error signal are shown separate from remaining part of the controller 2. The action in the controller 2 can be any control scheme suitable for the process to be controlled, such as PID (Proportional, Integral, Derivative control). The output signal from the controller 2 controls the desired process variable either directly or indirectly. For example, the controller 2 may position a control valve. Process changes, i.e. changes in the process variable resulting from the valve changes are measured by the measurement sensor/transmitter 6 and the procedure continues. All sections of the control system may be continuously connected, even though action does not take place unless a change takes place that results in the process variable measured deviating from the set point (desired control point).
Traditionally all connections in automation system have been provided by wired connections, e.g. cables. However, there is an increasing interest of using wireless sensor/transmitters in feedback loops in order to the need for expensive cabling of sensors. Use of wireless transmitters provides a new degree of flexibility in reconfiguring the process without installing or relocating transmitter wiring. There is also more freedom if the monitored process is far away from the controller or if the process is very wide, and it is difficult (and sometimes impossible) to use wired coupling.
Although the measurements are sent periodically from the sensor/transmitter, the same measurement may take different routes in the wireless network, which results in non-periodic reception of measurements and varying delay (jitter). Another typical feature is connection breaks, with lost packages. The controller must take into account that up to 100% of packets may be lost during (temporary) communications breaks. Otherwise, the closed-loop system may become unstable. One approach to address the problem of unreliable communications is a PID PLUS controller disclosed in U.S. Pat. No. 7,587,252, U.S. Pat. No. 7,620,460, and US2009/0299495, for example. The PID PLUS controller is capable of adapting to the non-periodic measurements by taking into account the time passed since the previous measurement update. More specifically the integral and derivative actions of the PID PLUS controller are calculated over the time interval between two consecutively received measurements. Thus, integral and derivative depend on the time between the previous received measurements and they are only calculated then a new measurement has arrived. If no new measurement is received, the output of the controller is constant. A disadvantage of this approach is that standard PID controller cannot be used.
Karl-Erik Årzén: “A Simple Event-Based PID Controller”. In Preprints 14th World Congress of IFAC, Beijing, P.R. China, January 1999, proposes an event-based PID in which integral (I) and derivative (D) controls are dependent on an instantaneous control interval. A problem of this approach is that it is not robust but the control becomes instable during long breaks in communication.
A networked PID and a steady-state heuristic PID controller are two theoretical approaches proposed by M. Björkbom, “Wireless Control System Simulation and Network Adaptive Control,” Ph.D. thesis, Control Engineering report 167, Aalto University, School of Science and Technology, October, 2010. In the networked PID approach a PID controller is split into two parts and distributed over the network such that part of the algorithm is at the sensor. Thus, a “smart sensor” with some computational abilities is needed. A reference signal (set point) is also needed. On the sensor side the error, integral error and derivative error are calculated and transmitted to the controller where the final control output is calculated. In this approach, the error estimates calculated at the sensor side are always exact. Whenever the controller receives update estimates from the sensor, the control output is correct. If no data from the sensor is received, the previously received values can be held. A disadvantage of this approach is that the sensor must be aware of setpoint, which is very problematic in practice. In the steady-state heuristic PID controller approach an approximate closed-loop step response is a rough estimate of the output behaviour, when the actual measurement feedback information is unavailable. Using this estimate, the control can continue to bring the process into a desired steady-state, although measurements are not updated. A disadvantage of this approach is that a process model is needed.